Sizing up a new measuring ruler for the solar system

At a recent meeting of the IAU in Beijing, China, members unanimously voted to redefine the astronomical unit, or AU, which has long served as the fundamental unit of distance between objects in the solar system. According to the voters, the official definition of the AU is now exactly 149,597,870,700 metres, and the unit should be written “au”.

Historically, calculating the astronomical unit was based on the average distance between Earth and the sun, or 149,597,870,691 metres. An amendment in 1976 complicated things by also tying the unit to the sun’s mass.

Although the recent decision doesn’t alter the value by much, it simplifies things and should improve the accuracy of distance measurements over time.

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“The old definition was good when we were not able to measure distance precisely in the solar system,” says Sergei Klioner of the Technical University of Dresden, Germany, who has been calling for the change since 2005.

With current technology, astronomers can measure distances directly with lasers and space probes, making it possible to give exact values in astronomical units.

Pain to explain

For the past 36 years the formal definition for the astronomical unit has been calculated using the Gaussian gravitational constant, a figure that depends on the mass of the sun. But astronomers know that the sun is constantly losing mass as it radiates energy, which technically changes the value of the AU over time.

Defining the unit as a set number fixes this problem and brings it in line with the effects of general relativity, says Klioner.

“The old definition was conceived in Newtonian physics,” he says. Under Einstein’s view of the universe, all distances are relative depending on where you are, making the Earth-sun distance meaningless without specifying your current reference frame. “If you use several reference frames, you have to ask which one the old AU is in.”

In addition, the metre itself is defined as the distance travelled by light in a vacuum in one-299,792,458th of a second. The speed of light is fixed in all reference frames and is unaffected by the changing mass of the sun, so giving the astronomical unit a set value in metres means that it will no longer waver.

“I’ve been teaching celestial mechanics for 20 years and it was always a pain to explain the old definition. It was clear that it was unnecessarily complicated,” he says. “I’m happy that I don’t have to explain this any longer.”